In a conventional DNA chip for electrochemically detecting a specific gene sequence, a single-stranded nucleic acid probe having a base sequence that is complementary to a target gene to be detected is immobilized on an electrode surface, and the nucleic acid probe and the target gene sample that is denatured to a single strand are hybridized, and thereafter, an intercalator which is electrochemically active and specifically binds to the double-stranded nucleic acid comprising the nucleic acid probe and the target gene sample is added to a reaction system for the nucleic acid probe and the gene sample, and then the intercalator bonded to the double-stranded nucleic acid is detected by performing electrochemical measurement through the electrode, whereby the nucleic acid probe that is hybridized with the target gene sample is detected to confirm existence of the target gene (for example, refer to Patent Document 1 (U.S. Pat. No. 2,573,443) and Patent Document 2 (U.S. Pat. No. 3,233,851)).
The intercalator indicates a substance that recognizes the double-stranded nucleic acid and specifically binds to the double-stranded nucleic acid. The intercalator has a tabular intercalation base such as phenyl in a molecule, and binds to the double-stranded nucleic acid by that the intercalation base is intercalated between a base pair and a base pair of the double-stranded nucleic acid. This binding of the intercalator and the double-stranded nucleic acid is a binding due to electrostatic interaction or hydrophobic interaction, and it is a binding caused by equilibrium reaction in which intercalation of the intercalator between the base pairs of the double-stranded nucleic acid and separation of the intercalator from between the base pairs are repeated at a constant speed.
Among the above-mentioned intercalators, there is a substance that causes electrically reversible oxidation-reduction reaction. By using such intercalator that causes electrochemically reversible oxidation-reduction reaction, it is possible to detect existence of the intercalator bonded to the double-stranded nucleic acid by measuring the electrochemical change. As an output signal of this electrochemical change, there is current or luminescence that occurs during the oxidation-reduction.
That is, in the conventional gene detection method, it is important that the intercalator is specifically bonded to only the double-stranded nucleic acid, and that the amount of the intercalator bonded to the double-stranded nucleic acid is accurately detected.
However, the intercalator used for the conventional gene detection is nonspecifically adsorbed to the single-stranded nucleic acid probe and to the electrode surface on which the nucleic acid probe is immobilized, due to chemical bonding such as coordinate bonding or covalent bonding, or electrostatic interaction or hydrophobic interaction. The nonspecifically adsorbed intercalator causes background noise when detecting the amount of the intercalator bonded to the double-stranded nucleic acid, leading to reduction in the detection sensitivity.
In order to solve this problem, in the above-mentioned detection method, it is necessary to perform a washing process for removing the intercalator that is nonspecifically adsorbed to the single-stranded nucleic acid probe and the electrode surface, after addition of the intercalator.